Effect of halogen substitution of indole-3-acetic acid on biological activity in pea fruit

Abstract

Auxins (a class of plant growth hormones naturally present in all plants) have been implicated in fruit growth of pea. Pea ( Pisum sativum L.) fruit contain the auxins indole-3-acetic acid (IAA) and 4-chloroindole-3-acetic acid (4-C1-IAA). Fruits grow poorly and subsequently abscise when seeds are removed two days after anthesis, but 4-C1-IAA can substitute for the seeds in maintaining growth of deseeded fruit (pericarp) in planta. Applications of 4-C1-IAA promoted pericarp growth, the effect increasing with concentration from 1 to 100 μM, but IAA was ineffective in stimulating growth when tested from 0.1 to 100 μM. The effect of the position of the halogen on pericarp growth was examined by assaying the activities of 4-, 5-, 6- and 7-chloro- and fluoro-substituted IAA. The position and type of halogen dramatically affected auxin activity, with the natural product 4-C1-IAA being most effective. Of the other compounds tested, only 5-C1-IAA stimulated pea pericarp elongation, and then only moderately. Fluoro-substituted IAAs did not stimulate pericarp growth, and 4-F-IAA was inhibitory. This study is unique in that it reports the biological activity of 4-C1-IAA and halogen-IAA analogues in tissues of intact plants known to contain 4-C1-IAA. The relative activity of the compounds is discussed in reference to previous reports of auxin activity in other systems, and 4-C1-IAA's possible importance in pea fruit growth.